NAME

DBIx::BatchChunker - Run large database changes safely

VERSION

version 0.93

SYNOPSIS

    use DBIx::BatchChunker;

    my $account_rs = $schema->resultset('Account')->search({
        account_type => 'deprecated',
    });

    my %params = (
        chunk_size  => 5000,
        target_time => 5,

        rs      => $account_rs,
        id_name => 'account_id',

        coderef => sub { $_[1]->delete },
        sleep   => 1,
        debug   => 1,

        process_name     => 'Deleting deprecated accounts',
        process_past_max => 1,
    );

    # EITHER:
    # 1) Automatically construct and execute the changes:

    DBIx::BatchChunker->construct_and_execute(%params);

    # OR
    # 2) Manually construct and execute the changes:

    my $batch_chunker = DBIx::BatchChunker->new(%params);

    $batch_chunker->calculate_ranges;
    $batch_chunker->execute;

DESCRIPTION

This utility class is for running a large batch of DB changes in a manner that doesn't cause huge locks, outages, and missed transactions. It's highly flexible to allow for many different kinds of change operations, and dynamically adjusts chunks to its workload.

It works by splitting up DB operations into smaller chunks within a loop. These chunks are transactionalized, either naturally as single-operation bulk work or by the loop itself. The full range is calculated beforehand to get the right start/end points. A progress bar will be created to let the deployer know the processing status.

There are two ways to use this class: call the automatic constructor and executor ("construct_and_execute") or manually construct the object and call its methods. See "SYNOPSIS" for examples of both.

DISCLAIMER: You should not rely on this class to magically fix any and all locking problems the DB might experience just because it's being used. Thorough testing and best practices are still required.

Processing Modes

This class has several different modes of operation, depending on what was passed to the constructor:

DBIC Processing

If both "rs" and "coderef" are passed, a chunk ResultSet is built from the base ResultSet, to add in a BETWEEN clause, and the new ResultSet is passed into the coderef. The coderef should run some sort of active ResultSet operation from there.

An "id_name" should be provided, but if it is missing it will be looked up based on the primary key of the ResultSource.

If "single_rows" is also enabled, then each chunk is wrapped in a transaction and the coderef is called for each row in the chunk. In this case, the coderef is passed a Result object instead of the chunk ResultSet.

Note that whether "single_rows" is enabled or not, the coderef execution is encapsulated in DBIC's retry logic, so any failures will re-connect and retry the coderef. Because of this, any changes you make within the coderef should be idempotent, or should at least be able to skip over any already-processed rows.

Active DBI Processing

If an "stmt" (DBI statement handle args) is passed without a "coderef", the statement handle is merely executed on each iteration with the start and end IDs. It is assumed that the SQL for the statement handle contains exactly two placeholders for a BETWEEN clause. For example:

    my $update_stmt = q{
    UPDATE
        accounts a
        JOIN account_updates au USING (account_id)
    SET
        a.time_stamp = au.time_stamp
    WHERE
        a.account_id BETWEEN ? AND ? AND
        a.time_stamp != au.time_stamp
    });

The BETWEEN clause should, of course, match the IDs being used in the loop.

The statement is ran with "dbi_connector" for retry protection. Therefore, the statement should also be idempotent.

Query DBI Processing

If both a "stmt" and a "coderef" are passed, the statement handle is prepared and executed. Like the "Active DBI Processing" mode, the SQL for the statement should contain exactly two placeholders for a BETWEEN clause. Then the $sth is passed to the coderef. It's up to the coderef to extract data from the executed statement handle, and do something with it.

If single_rows is enabled, each chunk is wrapped in a transaction and the coderef is called for each row in the chunk. In this case, the coderef is passed a hashref of the row instead of the executed $sth, with lowercase alias names used as keys.

Note that in both cases, the coderef execution is encapsulated in a DBIx::Connector::Retry call to either run or txn (using "dbi_connector"), so any failures will re-connect and retry the coderef. Because of this, any changes you make within the coderef should be idempotent, or should at least be able to skip over any already-processed rows.

DIY Processing

If a "coderef" is passed but neither a stmt nor a rs are passed, then the multiplier loop does not touch the database. The coderef is merely passed the start and end IDs for each chunk. It is expected that the coderef will run through all database operations using those start and end points.

It's still valid to include "min_stmt", "max_stmt", and/or "count_stmt" in the constructor to enable features like max ID recalculation or chunk resizing.

TL;DR Version

    $stmt                             = Active DBI Processing
    $stmt + $coderef                  = Query DBI Processing  | $bc->$coderef($executed_sth)
    $stmt + $coderef + single_rows=>1 = Query DBI Processing  | $bc->$coderef($row_hashref)
    $rs   + $coderef                  = DBIC Processing       | $bc->$coderef($chunk_rs)
    $rs   + $coderef + single_rows=>1 = DBIC Processing       | $bc->$coderef($result)
            $coderef                  = DIY Processing        | $bc->$coderef($start, $end)

ATTRIBUTES

See the "METHODS" section for more in-depth descriptions of these attributes and their usage.

DBIC Processing Attributes

rs

A DBIx::Class::ResultSet. This is used by all methods as the base ResultSet onto which the DB changes will be applied. Required for DBIC processing.

rsc

A DBIx::Class::ResultSetColumn. This is only used to override "rs" for min/max calculations. Optional.

dbic_retry_opts

A hashref of DBIC retry options. These options control how retry protection works within DBIC. So far, there are two supported options:

    max_attempts  = Number of times to retry
    retry_handler = Coderef that returns true to continue to retry or false to re-throw
                    the last exception

The default is to use DBIC's built-in retry options, the same way "dbh_do" in DBIx::Class::Storage::DBI does it, which will retry once if the DB connection was disconnected. If you specify any options, even a blank hashref, BatchChunker will fill in a default max_attempts of 10, and an always-true retry_handler. This is similar to DBIx::Connector::Retry's defaults.

Under the hood, these are options that are passed to the as-yet-undocumented DBIx::Class::Storage::BlockRunner. The retry_handler has access to the same BlockRunner object (passed as its only argument) and its methods/accessors, such as storage, failed_attempt_count, and last_exception.

DBI Processing Attributes

dbi_connector

A DBIx::Connector::Retry object. Instead of DBI statement handles, this is the recommended way for BatchChunker to interface with the DBI, as it handles retries on failures. The connection mode used is whatever default is set within the object.

Required for DBI Processing, unless "dbic_storage" is specified.

dbic_storage

A DBIC storage object, as an alternative for "dbi_connector". There may be times when you want to run plain DBI statements, but are still using DBIC. In these cases, you don't have to create a DBIx::Connector::Retry object to run those statements.

This uses a BlockRunner object for retry protection, so the options in "dbic_retry_opts" would apply here.

Required for DBI Processing, unless "dbi_connector" is specified.

min_stmt

max_stmt

SQL statement strings or an arrayref of parameters for "selectrow_array" in DBI.

When executed, these statements should each return a single value, either the minimum or maximum ID that will be affected by the DB changes. These are used by "calculate_ranges". Required if using either type of DBI Processing.

stmt

A SQL statement string or an arrayref of parameters for "prepare" in DBI + binds.

If using "Active DBI Processing" (no coderef), this is a do-able statement (usually DML like INSERT/UPDATE/DELETE). If using "Query DBI Processing" (with coderef), this is a passive DQL (SELECT) statement.

In either case, the statement should contain BETWEEN placeholders, which will be executed with the start/end ID points. If there are already bind placeholders in the arrayref, then make sure the BETWEEN bind points are last on the list.

Required for DBI Processing.

count_stmt

A SELECT COUNT SQL statement string or an arrayref of parameters for "selectrow_array" in DBI.

Like "stmt", it should contain BETWEEN placeholders. In fact, the SQL should look exactly like the "stmt" query, except with COUNT(*) instead of the column list.

Used only for "Query DBI Processing". Optional, but recommended for chunk resizing.

Progress Bar Attributes

progress_bar

The progress bar used for all methods. This can be specified right before the method call to override the default used for that method. Unlike most attributes, this one is read-write, so it can be switched on-the-fly.

Don't forget to remove or switch to a different progress bar if you want to use a different one for another method:

    $batch_chunker->progress_bar( $calc_pb );
    $batch_chunker->calculate_ranges;
    $batch_chunker->progress_bar( $loop_pb );
    $batch_chunker->execute;

All of this is optional. If the progress bar isn't specified, the method will create a default one. If the terminal isn't interactive, the default Term::ProgressBar will be set to silent to naturally skip the output.

progress_name

A string used by "execute" to assist in creating a progress bar. Ignored if "progress_bar" is already specified.

This is the preferred way of customizing the progress bar without having to create one from scratch.

cldr

A CLDR::Number object. English speakers that use a typical 1,234.56 format would probably want to leave it at the default. Otherwise, you should provide your own.

debug

Boolean. If turned on, displays timing stats on each chunk, as well as total numbers.

Common Attributes

id_name

The column name used as the iterator in the processing loops. This should be a primary key or integer-based (indexed) key, tied to the resultset.

Optional. Used mainly in DBIC processing. If not specified, it will look up the first primary key column from "rs" and use that.

This can still be specified for other processing modes to use in progress bars.

coderef

The coderef that will be called either on each chunk or each row, depending on how "single_rows" is set. The first input is always the BatchChunker object. The rest vary depending on the processing mode:

    $stmt + $coderef                  = Query DBI Processing  | $bc->$coderef($executed_sth)
    $stmt + $coderef + single_rows=>1 = Query DBI Processing  | $bc->$coderef($row_hashref)
    $rs   + $coderef                  = DBIC Processing       | $bc->$coderef($chunk_rs)
    $rs   + $coderef + single_rows=>1 = DBIC Processing       | $bc->$coderef($result)
            $coderef                  = DIY Processing        | $bc->$coderef($start, $end)

The loop does not monitor the return values from the coderef.

Required for all processing modes except "Active DBI Processing".

chunk_size

The amount of rows to be processed in each loop.

Default is 1000 rows. This figure should be sized to keep per-chunk processing time at around 5 seconds. If this is too large, rows may lock for too long. If it's too small, processing may be unnecessarily slow.

target_time

The target runtime (in seconds) that chunk processing should strive to achieve, not including "sleep". If the chunk processing times are too high or too low, this will dynamically adjust "chunk_size" to try to match the target.

Turning this on does not mean you should ignore chunk_size! If the starting chunk size is grossly inaccurate to the workload, you could end up with several chunks in the beginning causing long-lasting locks before the runtime targeting reduces them down to a reasonable size.

Default is 5 seconds. Set this to zero to turn off runtime targeting. (This was previously defaulted to off prior to v0.92, and set to 15 in v0.92.)

sleep

The number of seconds to sleep after each chunk. It uses Time::HiRes's version, so fractional numbers are allowed.

Default is 0, which is fine for most operations. But, it is highly recommended to turn this on (say, 1 to 5 seconds) for really long one-off DB operations, especially if a lot of disk I/O is involved. Without this, there's a chance that the slaves will have a hard time keeping up, and/or the master won't have enough processing power to keep up with standard load.

This will increase the overall processing time of the loop, so try to find a balance between the two.

process_past_max

Boolean that controls whether to check past the "max_id" during the loop. If the loop hits the end point, it will run another maximum ID check in the DB, and adjust max_id accordingly. If it somehow cannot run a DB check (no "rs" or "max_stmt" available, for example), the last chunk will check all the way to $DB_MAX_ID.

This is useful if the entire table is expected to be processed, and you don't want to miss any new rows that come up between "calculate_ranges" and the end of the loop.

Turned off by default.

NOTE: If your RDBMS has a problem with a number as high as whatever max_integer reports, you may want to set the $DB_MAX_ID global variable in this module to something lower.

single_rows

Boolean that controls whether single rows are passed to the "coderef" or the chunk's ResultSets/statement handle is passed.

Since running single-row operations in a DB is painfully slow (compared to bulk operations), this also controls whether the entire set of coderefs are encapsulated into a DB transaction. Transactionalizing the entire chunk brings the speed, and atomicity, back to what a bulk operation would be. (Bulk operations are still faster, but you can't do anything you want in a single DML statement.)

Used only by "DBIC Processing" and "Query DBI Processing".

min_chunk_percent

The minimum row count, as a percentage of "chunk_size". This value is actually expressed in decimal form, i.e.: between 0 and 1.

This value will be used to determine when to process, skip, or expand a block, based on a count query. The default is 0.5 or 50%, which means that it will try to expand the block to a larger size if the row count is less than 50% of the chunk size. Zero-sized blocks will be skipped entirely.

This "chunk resizing" is useful for large regions of the table that have been deleted, or when the incrementing ID has large gaps in it for other reasons. Wasting time on numerical gaps that span millions can slow down the processing considerably, especially if "sleep" is enabled.

If this needs to be disabled, set this to 0. The maximum chunk percentage does not have a setting and is hard-coded at 100% + min_chunk_percent.

If DBIC processing isn't used, "count_stmt" is also required to enable chunk resizing.

min_id

max_id

Used by "execute" to figure out the main start and end points. Calculated by "calculate_ranges".

Manually setting this is not recommended, as each database is different and the information may have changed between the DB change development and deployment. Instead, use "calculate_ranges" to fill in these values right before running the loop.

Private Attributes

_loop_state

These variables exist solely for the processing loop. They should be cleared out after use. Most of the complexity is needed for chunk resizing.

timer

Timer for debug messages. Always spans the time between debug messages.

start

The real start ID that the loop is currently on. May continue to exist within iterations if chunk resizing is trying to find a valid range. Otherwise, this value will become undef when a chunk is finally processed.

end

The real end ID that the loop is currently looking at. This is always redefined at the beginning of the loop.

prev_end

Last "processed" value of end. This also includes skipped blocks. Used in start calculations and to determine if the end of the loop has been reached.

max_end

The maximum ending ID. This will be $DB_MAX_ID if "process_past_max" is set.

last_range

A hashref of keys used for the bisecting of one block. Cleared out after a block has been processed or skipped.

last_timings

An arrayref of hashrefs, containing data for the previous 5 runs. This data is used for runtime targeting.

multiplier_range

The range (in units of "chunk_size") between the start and end IDs. This starts at 1 (at the beginning of the loop), but may expand or shrink depending on chunk count checks. Resets after block processing.

multiplier_step

Determines how fast multiplier_range increases, so that chunk resizing happens at an accelerated pace. Speeds or slows depending on what kind of limits the chunk count checks are hitting. Resets after block processing.

checked_count

A check counter to make sure the chunk resizing isn't taking too long. After ten checks, it will give up, assuming the block is safe to process.

chunk_size

The current chunk size, which might be adjusted by runtime targeting.

chunk_count

Records the results of the COUNT(*) query for chunk resizing.

prev_check

A short string recording what happened during the last chunk resizing check. Exists purely for debugging purposes.

prev_runtime

The number of seconds the previously processed chunk took to run, not including sleep time.

progress_bar

The progress bar being used in the loop. This may be different than "progress_bar", since it could be auto-generated.

CONSTRUCTORS

See "ATTRIBUTES" for information on what can be passed into these constructors.

new

    my $batch_chunker = DBIx::BatchChunker->new(...);

A standard object constructor. If you use this constructor, you will need to manually call "calculate_ranges" and "execute" to execute the DB changes.

construct_and_execute

    my $batch_chunker = DBIx::BatchChunker->construct_and_execute(...);

Constructs a DBIx::BatchChunker object and automatically calls "calculate_ranges" and "execute" on it. Anything passed to this method will be passed through to the constructor.

Returns the constructed object, post-execution. This is typically only useful if you want to inspect the attributes after the process has finished. Otherwise, it's safe to just ignore the return and throw away the object immediately.

METHODS

calculate_ranges

    my $batch_chunker = DBIx::BatchChunker->new(
        rsc      => $account_rsc,    # a ResultSetColumn
        ### OR ###
        rs       => $account_rs,     # a ResultSet
        id_name  => 'account_id',    # can be looked up if not provided
        ### OR ###
        dbi_connector => $conn,      # DBIx::Connector::Retry object
        min_stmt      => $min_stmt,  # a SQL statement or DBI $sth args
        max_stmt      => $max_stmt,  # ditto

        ### Optional but recommended ###
        id_name      => 'account_id',  # will also be added into the progress bar title
        chunk_size   => 20_000,        # default is 1000

        ### Optional ###
        progress_bar => $progress,     # defaults to a 2-count 'Calculating ranges' bar

        # ...other attributes for execute...
    );

    my $has_data_to_process = $batch_chunker->calculate_ranges;

Given a DBIx::Class::ResultSetColumn, DBIx::Class::ResultSet, or DBI statement argument set, this method calculates the min/max IDs of those objects. It fills in the "min_id" and "max_id" attributes, based on the ID data, and then returns 1.

If either of the min/max statements don't return any ID data, this method will return 0.

execute

    my $batch_chunker = DBIx::BatchChunker->new(
        # ...other attributes for calculate_ranges...

        dbi_connector => $conn,          # DBIx::Connector::Retry object
        stmt          => $do_stmt,       # INSERT/UPDATE/DELETE $stmt with BETWEEN placeholders
        ### OR ###
        dbi_connector => $conn,          # DBIx::Connector::Retry object
        stmt          => $select_stmt,   # SELECT $stmt with BETWEEN placeholders
        count_stmt    => $count_stmt,    # SELECT COUNT $stmt to be used for min_chunk_percent; optional
        coderef       => $coderef,       # called code that does the actual work
        ### OR ###
        rs      => $account_rs,          # base ResultSet, which gets filtered with -between later on
        id_name => 'account_id',         # can be looked up if not provided
        coderef => $coderef,             # called code that does the actual work
        ### OR ###
        coderef => $coderef,             # DIY database work; just pass the $start/$end IDs

        ### Optional but recommended ###
        sleep             => 0.25, # number of seconds to sleep each chunk; defaults to 0
        process_past_max  => 1,    # use this if processing the whole table
        single_rows       => 1,    # does $coderef get a single $row or the whole $chunk_rs / $stmt
        min_chunk_percent => 0.25, # minimum row count of chunk size percentage; defaults to 0.5 (or 50%)
        target_time       => 5,    # target runtime for dynamic chunk size scaling; default is 5 seconds

        progress_name => 'Updating Accounts',  # easier than creating your own progress_bar

        ### Optional ###
        progress_bar     => $progress,  # defaults to "Processing $source_name" bar
        debug            => 1,          # displays timing stats on each chunk
    );

    $batch_chunker->execute if $batch_chunker->calculate_ranges;

Applies the configured DB changes in chunks. Runs through the loop, processing a statement handle, ResultSet, and/or coderef as it goes. Each loop iteration processes a chunk of work, determined by "chunk_size".

The "calculate_ranges" method should be run first to fill in "min_id" and "max_id". If either of these are missing, the function will assume "calculate_ranges" couldn't find them and warn about it.

More details can be found in the "Processing Modes" and "ATTRIBUTES" sections.

PRIVATE METHODS

_process_block

Runs the DB work and passes it to the coderef. Its return value determines whether the block should be processed or not.

_process_past_max_checker

Checks to make sure the current endpoint is actually the end, by checking the database. Its return value determines whether the block should be processed or not.

See "process_past_max".

_chunk_count_checker

Checks the chunk count to make sure it's properly sized. If not, it will try to shrink or expand the current chunk (in chunk_size increments) as necessary. Its return value determines whether the block should be processed or not.

See "min_chunk_percent".

This is not to be confused with the "_runtime_checker", which adjusts chunk_size after processing, based on previous run times.

_runtime_checker

Stores the previously processed chunk's runtime, and then adjusts chunk_size as necessary.

See "target_time".

_increment_progress

Increments the progress bar.

_print_debug_status

Prints out a standard debug status line, if debug is enabled. What it prints is generally uniform, but it depends on the processing action. Most of the data is pulled from "_loop_state".

SEE ALSO

DBIx::BulkLoader::Mysql, DBIx::Class::BatchUpdate, DBIx::BulkUtil

AUTHOR

Grant Street Group <developers@grantstreet.com>

LICENSE AND COPYRIGHT

Copyright 2018 Grant Street Group

This program is free software; you can redistribute it and/or modify it under the terms of the the Artistic License (2.0). You may obtain a copy of the full license at:

http://www.perlfoundation.org/artistic_license_2_0